Process for making electrical feedthroughs for ceramic circuit board support substrates

ABSTRACT

Electrical feedthroughs in printed circuit board support substrates for use in making double sided ceramic multilayer printed circuit boards are made by insulating the feedthrough openings with a first layer of nickel oxide and one or more layers of glass, and then filling the remainder of the feedthroughs with a conductive metal via fill ink. After firing, the resultant structure provides insulated electrical feedthroughs through the support substrate.

This invention was made with Government support under DAAB07-94-C-C009awarded by the U.S. Department of the Army. The Government has certainrights in this invention.

This is a division of application Ser. No. 08/379,264 filed 27 Jan.1995, now U.S. Pat. No. 5,565,262, Oct. 15, 1996.

This invention relates to a method of making electrical feedthroughs inthermally conductive support substrates used to impart mechanicalstrength to ceramic multilayer printed circuit boards. Moreparticularly, this invention relates to a method of making electricalfeedthroughs in ceramic multilayer printed circuit board supportsubstrates that is compatible with mass production techniques.

BACKGROUND OF THE INVENTION

Ceramic multilayer printed circuit boards have been used for many yearsfor circuits for electrical apparatus, such as mainframe computers. Suchprinted circuit boards are made by casting glass and/or ceramic powderstogether with an organic binder into tapes, called green tapes. A metalcircuit can be patterned onto the green tape by screen printing forexample. Vias are formed in each green tape layer that are filled with aconductive material to connect the circuits of the various layerselectrically. The green tape layers are then aligned and stacked,pressed together, and fired to burn off organic residues and sinter theglass, thereby forming a fired ceramic multilayer circuit board.

Originally ceramics such as alumina were used to form the green tapelayers, but these ceramics require high firing temperatures, up to 1500°C. This necessitated the use of refractory conductive metals, such astungsten or molybdenum, to form the conductive circuit .patterns becausesuch metals could withstand high firing temperatures without melting.More recently, lower temperature materials have been used, such asdevitrifying glasses that can be fired at lower temperatures of 1000° C.or less. Multilayer circuit boards made of these glass or glass-ceramicmaterials can be used with lower melting point and higher conductivitymetals, such as silver, gold or copper. However, these printed circuitboards have the disadvantage that they are not as strong as aluminacircuit boards.

Thus still more recently, low firing temperature glasses have beendeposited on support substrates made of metal or ceramic to which theglasses will adhere. The support substrate can be of a thermallyconductive material such as nickel, kovar, aferrous/nickel/cobalt/maganese alloy, Invar®, a ferronickel, low carbonsteel, or Cu/kovar, a ferrous/nickel/cobalt/maganese alloy/Cu, Cu/Mo/Cuor Cu/Invar®/Cu composites and the like, as well as thermally conductiveceramics such as aluminum nitride, silicon carbide, diamond and thelike. These substrates impart added strength to the composite. A bondingglass, such as described in U.S. Pat. No. 5,277,724 to Prabhu, adheresthe ceramic substrate formed from the green tape layers to thesubstrate. In addition, if chosen correctly, the bonding glass canreduce shrinkage of the green tape with respect to the metal substratein at least the two lateral dimensions. Thus all of the shrinkage occursin the thickness dimension only. This in turn reduces problems ofalignment of the circuit patterns in the ceramic layers, and the viaholes in the metal substrate after firing.

However, when it is desired to produce glass/ceramic multilayer ceramiccircuit boards on both sides of the support substrate, the presence ofthe thermally and electrically conductive metal or ceramic core materialbetween two circuit boards can cause short circuits. Thus the multilayercircuits on one side of the support substrate have been connected to themultilayer circuits on the other side of the support substrate by meansof circuit traces or lines that extend around the periphery of thecircuit board rather than through the support substrate. However, suchperipheral traces are subject to damage or breakage during handling andassembly of the circuit boards into a module, for example, and in somecases the traces would have to be too long for an acceptable design.Such designs also increase wiring lengths and decrease interconnectiondensity. Thus an improved method of permitting electrical connectionbetween two ceramic multilayer circuit boards on both sides of a supportsubstrate would be highly desirable.

SUMMARY OF THE INVENTION

The present process for forming electrical feedthroughs in supportsubstrates for double sided printed circuit board substrates comprisesproviding dielectric insulation in the feedthroughs. Typically a viahole is opened in the support substrate core material, as by drilling,the substrate via hole is plated with nickel, one or more dielectricmaterials such as glass is deposited in the via hole. Lastly aconductive metal is deposited to fill the via hole inside the dielectricring. The dielectric material and the center conductive metal must beable to withstand several firings at temperatures up to at least 900° C.without melting or flowing.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a flow chart of the preferred process for filling via holes ina printed circuit board support substrate in accordance with the processof the invention.

FIG. 2 is a thermal coefficient of expansion plot of a glass suitablefor use as a dielectric in the present process.

FIG. 3 is a differential thermal analysis (DTA) plot of a glass suitablefor use as a dielectric in the present process.

FIGS. 4A and 4B illustrate the steps of forming a glass dielectric layerin a via hole.

FIG. 5 is a cross sectional partial view of a printed circuit boardsupport substrate having a filled via hole filled in accordance with themethod of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The preferred support substrate for use herein is a Cu/Mo/Cu metalcomposite substrate commercially available from the Climax MetalsCompany, although other materials can be substituted, as describedhereinabove.

Referring to FIG. 1, which is a flow chart of a suitable process formaking the electrical feedthroughs in a printed circuit board supportsubstrate in accordance with the invention, in a first step of thepresent process, via openings can be formed in the support substrateusing a laser or mechanical drilling equipment that can drill smalldiameter holes, e.g., about 13-40 mils in diameter or less. Themechanically drilled openings are then deburred, as by rubbing the edgeswith a soft stone, whereby via openings having sharp corners areeliminated. The thicker the substrate material, the more difficulty maybe encountered in drilling the openings. Thirteen mil diameter holes canalso be readily drilled using a Nd:YAG laser at 15-30 watts with 0.6msec pulse lengths. A minimum hole diameter of 7 mils for a 20 mil thicksupport substrate can be made readily. If the thickness of the supportsubstrate is higher, the minimum hole diameter that can be made may belarger; for example, for a 40 mil thick support substrate, the minimumhole diameter that can be readily made is 8 mils.

The drilled holes are next deburred and nickel plated. This step sealsthe core material of the support substrate and can be accomplished byconventional nickel electroplating methods. The nickel is then oxidized,as by heating in air at temperatures about 820° C. The nickel oxidelayer, which exhibits a resistance of 10⁸ -10⁹ ohms, constitutes thefirst ring of dielectric material in the via hole.

An insulating dielectric layer, as of a glass, is then deposited in thevia hole to form an annular ring. Since glass is a fragile material thatcan crack during multiple firings, it is preferred that two or morelayers of glass be sequentially deposited in the openings so that if adefect, such as a pore, forms in one layer, it will not extend throughthe entire glass layer, to cause a shorted feedthrough.

The glasses suitable for use in the present invention, using Cu/Mo/Cumetal composite substrates, must have a thermal coefficient of expansionmatched to the Cu/Mo/Cu substrate; must have good adhesion to nickeloxide, must be able to wet nickel oxide; and must be able to be fired attemperatures required to form the desired ceramic multilayer circuitboard.

One particular glass composition having the following composition inpercent by weight is particularly useful with the above nickel platedCu/Mo/Cu composite metal substrate;

    ______________________________________                                                ZnO   28.68                                                                   MgO   5.92                                                                    BaO   6.21                                                                    Al.sub.2 O.sub.3                                                                    15.36                                                                   SiO.sub.2                                                                           43.82                                                           ______________________________________                                    

This glass has a thermal coefficient of expansion plot as shown in FIG.2 and a DTA plot as shown in FIG. 3. This glass can be used as thedielectric insulator for the substrate via holes. The same glass canalso be used later in the process as a constituent of the thick filmconductor via fill ink required for filling the center of each via holewith conductive metal, as further described below.

Another suitable glass composition for use with the preferred metalsubstrate has the following composition in percent by weight:

    ______________________________________                                                MgO   29.0                                                                    Al.sub.2 O.sub.3                                                                    22.0                                                                    SiO.sub.2                                                                           45.0                                                                    P.sub.2 O.sub.5                                                                     1.5                                                                     B.sub.2 O.sub.3                                                                     1.0                                                                     ZrO.sub.2                                                                           1.5                                                             ______________________________________                                    

A preferred method of applying the glass composition from a standardglazing ink constituting the above glass is to apply vacuum after thescreen printing to deposit one or more of the above glass layers. Such aglazing ink comprises the finely divided glass and an organic vehicle.Suitable organic vehicles are solutions of resin binders, such ascellulose derivatives, synthetic resins such as polyacrylates,polymethacrylates, polyesters, polyolefins and the like, in a suitablesolvent. The solvent can be pine oil, terpineol, butyl carbitol acetate,2,2,4-trimethyl-1,3-pentanediol monoisobutyrate and the like. Thevehicles generally contain from about 5 to 25 percent by weight of theresin binder.

FIG. 4A illustrates a printed glass layer 20 over a via hole 22 in ametal substrate 24.

A vacuum is applied after the printing, beneath the metal substrate 24in the direction of the arrow 25, sufficient to bring the glass inklayer 20 into the via hole 22, thereby forming an annular ring of theglass ink inside the via hole 22. This glass layer is then dried. Thedeposition and vacuum pull can be repeated to form multiple glassdielectric layers in the via hole 22. If both sides of the metalsubstrate 24 are to be used, the above sequence of steps is repeated onthe opposite side of the metal substrate 24.

The support substrate is then fired to sinter the glass powder and forma composite fired glass insulator layer in the opening.

A thick via fill ink containing a conductive metal powder is thenapplied to the metal substrate, also using conventional screen printingtechniques. For example, a suitable conductor thick film ink comprises amixture of silver or other conductive metal powder, glass, and anorganic vehicle as described above in proportions so as to form a printscreenable thick film paste.

Thick film conductor via fill inks are made by mixing a finely dividedconductive metal powder, with a preselected glass powder and an organicvehicle. Suitable conductive powders include silver, gold, copper, theirmixtures, and alloys thereof with palladium and platinum and the like,or nickel. The fired thick film conductive metal ink can comprise fromabout 50-90% by weight of metal and about 10-50% by weight of a glass.

The thick film conductor via fill ink composition is applied to theprepared printed circuit board support substrate so as to fill the glassinsulated via holes and is then fired to remove organic materials and tosinter the metal powder to obtain the conductive, insulatedfeedthroughs.

FIG. 5 is a cross sectional view of the metal substrate 24 havingdielectric insulated electrical feedthroughs therein. The via hole 22 inthe metal substrate 24 has a first layer 23 of nickel oxide dielectric,two dielectric glass layers 26, 28 and a conductive via fill layer 30therein. Sufficient conductive via fill ink is applied so that theremainder of the via hole 22 is completely filled at the end of theprocess.

The support substrate as prepared above, having conductive vias in viaopenings that are dielectrically insulated from the rest of thesubstrate, can then be used to prepare double sided multilayer printedcircuit boards from the substrates of the invention in conventionalmanner.

The above process can be used to make a reproducible support substratehaving a plurality of electrical feedthroughs therein that will not formshort circuits between circuitry on both sides of the substrate. Thesupport substrate having electrical feedthroughs as prepared above canwithstand several firings at temperatures used in making ceramicmultilayer printed circuit boards without undermining the structural andelectrical integrity of the feedthroughs.

Although the present process and electrical feedthroughs have beendescribed in terms of specific embodiments, one skilled in the art canreadily substitute other materials and reaction conditions for the glasslayers and conductors described hereinabove. Thus the scope of thepresent invention is only meant to be limited by the appended claims.

We claim:
 1. A process for making electrical feedthroughs in a printedcircuit board support substrate for use in making double sided ceramicmultilayer circuit boards comprising:a) forming via holes in saidsubstrate; b) forming an electrically insulating layer in said viaholes, c) applying a thick film conductor ink containing conductivemetal and glass powders in an organic vehicle to fill said via holes andd) firing said substrate to remove organic materials and to sinter themetal and glass powders in said via holes.
 2. A process according toclaim 1 wherein said glass has a thermal coefficient of expansionclosely matched to said substrate.
 3. A process according to claim 1wherein said substrate is a copper/molybdenum/copper composite.
 4. Aprocess according to claim 1 wherein said substrate is a thermallyconductive material selected from the group consisting of a ferronickel,a ferrous/nickel/cobalt/manganese alloy, copper/molybdenum/coppercomposite, and composites of copper clad conductive metals.
 5. A processaccording to claim 1 wherein said conductive metal is silver.
 6. Aprocess according to claim 1 wherein said conductive metal is selectedfrom the group consisting of copper, gold, mixtures thereof, alloysthereof with palladium or platinum, and nickel.
 7. A process for formingan electrically insulating layer for making electrical feedthroughs in aprinted circuit board support substrate for use in making double sidedceramic multilayer circuit boards comprising:a) forming via holes insaid substrate; b) electroplating said holes with nickel; c) heating thenickel coated substrate in an oxygen-containing atmosphere to form anickel oxide layer over said nickel coating; d) screen printing adielectric glass composition comprising glass and an organic vehicleover said via hole; e) applying a vacuum to draw a portion of said glasscomposition over said nickel oxide layer in the via hole; and f) firingsaid substrate to remove organic materials and to reflow the glass.